1. Field of the Invention
The present invention relates to an inkjet recording head and an inkjet recording apparatus, and in particularly, relates to a maintenance technology for an inkjet recording head, which performs recording by discharging ink from a nozzle onto a recording medium.
2. Description of the Related Art
Inkjet recording apparatuses (inkjet printers) are widely used because the inkjet recording apparatuses are relatively inexpensive, simple to handle, and capable of producing images of high quality. A commonly used inkjet recording apparatus comprises an inkjet recording head (a print head) having a plurality of nozzles, and discharges ink from the nozzles onto a record paper to perform printing. Hence, if the viscosity of the ink increases or the ink hardens due to the evaporation of the solvent or the like, then blocking of the nozzles may occur and it may become difficult to achieve the ink discharge. In the light of the circumstances, various proposals have been made with a view to preventing the blocking of nozzles by improving the ink, namely, by adding an agent for moisture retention to a water-based ink, and using a high-boiling-point solvent in the case of an oil-based ink.
If ink of this kind is used, then when printing onto a record paper of low permeability is performed, the ink takes a long time to dry. Hence, the bleeding and color mixture of the ink may arise and a printed surface being still wet may touch components of the main unit of the printer, consequently, the printed image may deteriorate in quality. In view of preventing such the deterioration of the image quality, Japanese Patent Application Publication No. 60-132767 discloses an inkjet recording apparatus in which ultraviolet-curable ink is used and the ink is caused to set by irradiating ultraviolet light immediately after discharging the ink onto the record paper.
Moreover, in view of preventing blocking of the nozzles, Japanese Patent Application Publication No. 2000-301730 discloses an inkjet recording apparatus in which the nozzles are sealed by a sealing liquid containing a coloring agent of a similar color hue to the ink discharged from the nozzles, in such a manner that drying of the ink is prevented. A method for preventing increase in viscosity of the ink by capping the meniscus surface by means of the sealing liquid in this way does not incur the wasteful ink consumption and the reduced printing speed that are described below. However, adverse effects on ink discharge and ink composition may result because the ink droplets make contact with the sealing liquid when they are discharged from the nozzles onto the recording medium. Hence, in Japanese Patent Application Publication No. 2000-301730, effects of this kind are prevented by using a sealing liquid containing a coloring agent of a similar color hue to the ink.
Furthermore, the nozzle is filled with ink at all times in order that printing can be implemented immediately whenever a printing command is issued. Hence, at the meniscus surface of the ink in the vicinity of the opening section of the nozzle, the solvent in the ink is liable to evaporate and hence the ink is liable to increase in viscosity. If the ink at the meniscus surface reaches a state of increased viscosity, then this may incur the discharge defects of the nozzles. If the evaporation continues further, then the ink component becomes a film-like form at the meniscus surface, and thereby it incurs discharge defects that are even harder to recover. Thus, the meniscus surface should be kept at all times in a state that does not incur an increase in viscosity.
Hence, a device for mechanically capping the nozzles, or the like, is used in order to prevent increase in viscosity of ink at the meniscus surface, in the case where a printing operation is not carried out for a long period of time, for instance, if the power supply is switched off.
On the other hand, in order to be able to carry out printing immediately if a print command is issued; it is preferable for the nozzles to be in an uncapped state during printing or during standby. Even if an inkjet recording head is provided with a plurality of nozzles, not all of the nozzles necessarily discharge ink during a printing operation, and hence the nozzle having a low ink discharge frequency may arise depending on the image data used to perform printing. In the case of a nozzle having a low ink discharge frequency, which continues in a state of not discharging ink for a certain period of time or more, the solvent in the ink in the vicinity of the meniscus surface evaporates and the viscosity of the ink increases. When a state of this kind occurs, it is difficult to discharge ink from the nozzle and hence discharge defects of the nozzles may occur.
Hence, in order to prevent discharge defects at a nozzle due to the increase in viscosity of the ink at the meniscus surface, a refresh operation, such as a preliminary discharge (e.g., “purge”, “blank discharge”, “liquid discharge”, or the like), is carried out periodically so that the degraded ink with increased viscosity is discharged. However, while the refresh operation is effective in preventing discharge defects in nozzles due to the ink with increased viscosity in the vicinity of the meniscus surface, the refresh operation requires consuming ink wastefully. Moreover, printing cannot be carried out during the refresh operation, and hence printing speed is reduced. If the number of refresh operations is lowered in order to maintain printing speed, then it may be difficult to satisfactorily prevent the increased viscosity of the ink at the meniscus surface.
Furthermore, in order to prevent nozzle blockages due to the increased viscosity of the ink, an ink vibration method is also known in which the meniscus of the ink is caused to vibrate by a piezoelectric element or the like. This method is able to suppress the wasteful consumption of ink, but it requires control for vibrating the meniscus surface and may be inherently unsuited to prevent the discharge defects.
Hence, Japanese Patent Application Publication No. 2003-191470 discloses an inkjet recording head that supplies a moisture retention liquid or ink to the meniscus surface, in order to prevent increase in viscosity of the ink at the meniscus surface when the mechanical capping of the nozzles is removed. In the inkjet recording head, an orifice plate formed with a nozzle is made from a porous member which can be impregnated with ink, and by supplying the moisture retention liquid or ink to the porous member, the ink in the vicinity of the meniscus is kept in a wet state and increase in viscosity of the ink at the meniscus surface is prevented. Furthermore, the inkjet recording head is also designed to prevent increase in viscosity of the ink at the meniscus surface by periodically suctioning the ink with increased viscosity in the vicinity of the meniscus surface via the orifice plate.
However, in the inkjet recording apparatus using conventional ultraviolet-curable type ink as described in Japanese Patent Application Publication No. 60-132767, in order to suppress the image deterioration, such as bleeding of the image or spreading of the dots, it is desirable to irradiate ultraviolet light at the earliest possible timing after discharging the ink, and hence it requires to position the ultraviolet irradiation source as closely as possible to the nozzle. However, if the light source is positioned close to the nozzle, then the luminous energy of scattered ultraviolet light arriving at the nozzle increases and hence the ink inside the nozzle is liable to harden and cause a nozzle blockage.
Moreover, in the inkjet recording apparatus described in Japanese Patent Application Publication No. 2000-301730, drying of the ink is prevented by sealing the nozzle with the sealing liquid, but it is difficult to prevent the ultraviolet-curable ink inside the nozzle from hardening due to the scattered ultraviolet light.
In this way, there still has not been proposed a technique that completely shields the ink inside the nozzle from the scattered light when the radiation-curable ink, such as an ultraviolet-curable ink, is used. Hence, there is a requirement to develop an inkjet recording apparatus that prevents nozzle blockages due to scattered light or the like when a radiation-curable ink is used.
Furthermore, in the inkjet recording head described in Japanese Patent Application Publication No. 2003-191470, the supply of the moisture retention liquid or the like to the meniscus surface is performed on the basis of capillary action. Therefore, when the use frequency of a nozzle is high, the moisture retention liquid or the like seeps out to the meniscus surface via the porous member of the orifice plate immediately after the ink discharge, and hence sufficient moisture retention liquid or the like is supplied to the meniscus and the meniscus surface can be maintained sufficiently in a wet state. However, if the use frequency of the nozzle falls, then the fluidity of the ink declines, the supply of moisture retention liquid or the like to the meniscus surface by means of capillary action through the porous member become more difficult, and it is difficult to maintain the meniscus surface in a wet state. Hence, the ink at the meniscus surface evaporates and becomes a state of increased viscosity, which may lead to discharge defects at the nozzle.
Furthermore, concerning the inkjet recording head described above, a method is proposed in which, if the meniscus surface increases in viscosity due to the decline in fluidity of the ink, then the ink with the increased viscosity at the meniscus surface is suctioned periodically via the orifice plate. However, similarly to the above-mentioned refresh operation, such as preliminary discharge, printing may not be performed during the suctioning of the ink and hence the printing speed may decline. In other words, if the number of the suctioning operations is reduced in order to ensure printing speed, it is difficult to sufficiently prevent increase in viscosity of the ink at the meniscus surface.
In this way, no inkjet recording head has yet been proposed which comprises a viscosity-increase-preventing-device giving a good wetting effect to the ink at the meniscus surface even when the fluidity of the ink has declined. In the light of the circumstances, there is a requirement to develop a viscosity-increase-preventing-device that provides a wetting effect for the ink at all times.